This invention relates to nuclear magnetic resonance imaging methods and systems and, more particularly, to the measurement of a pressure wave along a blood vessel using a magnetic resonance imaging (MRI) system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated. This signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Aortic stiffness appears to be a correlate of age, fitness, and coronary artery disease. It has been shown to influence left ventricular afterload and is an important variable in the management of ventricular disease. It may also be an early indicator of the presence of atherosclerotic disease as well as a predictor of the likelihood of aneurysmal rupture. A rapid and noninvasive technique for determining aortic distensibility is therefore desirable.
A number of techniques have been proposed for the determination of aortic stiffness, some based on the measurement of variations in aortic diameter and blood pressure over the cardiac cycle and others based on measuring the propagation velocity of a pressure or flow wave along the aorta. One technique described by C. J. Hardy, et al., "A One-Dimensional Velocity Technique for NMR Measurement of Aortic Distensibility", MRM 31:513-520 (1994), uses NMR excitation of a cylinder or "pencil" of spins aligned with the aorta to produce M-mode phase-contrast aortic blood-flow images, with cardiac gating and data interleaving employed to increase the effective time resolution. For some patients with weak blood flow or irregular heartbeat, however, this method can produce large measurement uncertainties.
A similar but more robust cardiac gated NMR technique for determining aortic distensibility is disclosed by C. J. Hardy, et al., "Pencil Excitation With Interleaved Fourier Velocity Encoding: NMR Measurement of Aortic Distensibility", MRM 35:814-819 (1996). An NMR pencil-excitation pulse is used here also, with a bipolar velocity-encoding gradient followed by a readout gradient applied along the pencil axis. Data interleaving is employed to improve the effective time resolution so that rapid propagation of wavefronts can be followed. In this method, the bipolar gradient is stepped through a range of values, with a Fourier transform applied to produce velocity distribution profiles for different phases of the heart cycle. If a sinusoidal bipolar gradient is employed which has maximum amplitude G and separation between lobe centers of T, then the velocity resolution V.sub.res obtained by this method is ##EQU1## where .gamma. is the gyromagnetic ratio. The resulting velocity distributions can be produced as a series of image frames in which the velocity wave resulting from the pressure wave can be seen propagating along the aorta. The position of the "foot" of this wave can be measured at successive image frames and used to determine the wave velocity (C). The wave velocity C and the density p of the blood in the vessel can then be used to determine the vessel distensibility D according to the relation EQU D=1/.rho.C.sup.2, Eq.2
where distensibility is defined as the fractional change in vessel cross-sectional area per unit change in blood pressure. For an incompressible fluid in a stiff vessel, pressure changes are instantaneously transmitted down the vessel, but for a vessel with compliant walls, the pressure wave distends the vessel, and travels along the vessel at a finite velocity.
The measurement of the location of the foot of the velocity wave in each image frame can be both tedious and subjective. An automated method is needed for calculating wave-velocity accurately from a series of image frames.